A world like ours, maybe, or too smart to say

Ken Croswell

ASTRONOMERS have discovered what may be five planets orbiting Tau Ceti, the closest single star beyond our solar system, the temperature and luminosity of which nearly match the sun's. If they are planets, one is about the right distance from the star to feature mild temperatures, oceans of water, and even life. Don't pack your bags just yet, though: the discovery still needs to be confirmed.

Tau Ceti is only 12 light-years from Earth, just three times as far as our sun's nearest stellar neighbour, Alpha Centauri. Tau Ceti resembles the sun so much that astronomer Frank Drake, who has long sought radio signals from possible extraterrestrial civilisations, made it his first target in 1960. Unlike most stars, which are faint, cool and small, Tau Ceti is a bright G-type yellow main-sequence star, like the sun, a trait that only one in 25 stars boasts. Moreover, unlike Alpha Centauri, which also harbours a G-type star and even a planet, Tau Ceti is single, so there's no second star in the system whose gravity could yank planets away.

Astronomer Mikko Tuomi of the University of Hertfordshire in Britain, and colleagues, analysed more than 6000 observations of Tau Ceti from telescopes in Chile, Hawaii and Siding Spring near Coonabarabran. As the researchers will report in Astronomy & Astrophysics, slight changes in Tau Ceti's motion through space suggest that the star may be responding to gravitational tugs from five planets that are only about two to seven times as massive as Earth.

Tau Ceti emits only 45 per cent as much light as the sun, so each planet receives less warmth than a planet would at the same distance from our sun. Tau Ceti's three innermost planets are probably too hot to support life. The farthest of the three is about as close to Tau Ceti as Mercury is to the sun.

It's the fourth planet that the scientists suggest might be another life-bearing world. If you lived there, you'd see a yellow sun in the sky, but your year would last just 168 days. That's because Tau Ceti is somewhat closer to its star than Venus is to the sun and thus revolves faster than Earth. The fifth and outermost planet completes an orbit every 640 days and is slightly closer to its star than Mars is to the sun.

However, Dr Tuomi's team warns that disturbances on the star itself may be producing the small velocity changes in Tau Ceti. Team member Professor Chris Tinney, an astronomer at the University of NSW, acknowledges the problem. ''It's certainly very tantalising evidence for potentially a very exciting planetary system,'' Tinney says. Verifying the discovery may take 10 years, and the scientists didn't want to wait. ''We felt that the best thing to do was to put the result out there and see if somebody can either independently confirm it or shoot it down.''

If the planets exist, they orbit a star about twice as old as our own, so a suitable planet has had plenty of time to develop life much more advanced than Homo sapiens. That may just explain why no one from Tau Ceti has ever contacted beings as primitive as us.